Various systems for extracting liquid and/or for compacting solid matters present in a solid-liquid material are known from the prior art. Such systems are used inter alia for the treatment of pulps resulting from paper industries, waste water treatment, mineral processing, agriculture and food processing, fisheries, breweries, wineries, chemical processing, oil and tar sand industry, etc. . . .
Known batch systems perform only filtration by using the feed pressure to force the liquid out through the mass of solid-liquid material and a filtering means. By combining filtration and mechanical pressure, a dryer cake is recovered.
Such batch systems as those described in patent U.S. Pat. No. 4,565,602, are successfully used in the de-watering treatment of low-consistency slurries containing fine particles but are not suited for the treatment of pastes solid-liquid materials containing large particles.
In order to overcome the above-mentioned limitations of the batch systems and in order to improve the efficiency of dewatering plants, continuous systems have been developed. For example U.S. Pat. No. 4,534,868 describes a U-channel rotary press allowing the treatment of pastes and fibrous solid-liquid material. But this system was found not to be appropriate for the treatment of slurries containing fine particles, particularly for pastes containing fibrous materials.
Further continuous systems which are improvements of the system according to U.S. Pat. No. 4,534,868 are described in U.S. Pat. No. 4,986,881, U.S. Pat. No. 4,534,868 and U.S. Pat. No. 5,213,686. However, those systems still present limitations in a continuous use since the structure and particularly the size and the form of the constituting elements of the system should be carefully adapted to the specific composition and/or consistency of the liquid-solid material to be treated otherwise blockage of the system occurs and poor yields are unavoidable.
U.S. Pat. No. 5,205,941 describes a dewatering system having a number of rotors rotable in a cylindrical press housing with an inlet channel defining an inlet for a humid mass into the housing, a mechanism for equalising the amount of the mass fed to the inlet channel. A screw feeder is associated with each inlet channel, and equalization is accomplished by providing opening communication between the screw feeders at their pressure ends, adjacent the inlet channels. A module having a conduit associated with each inlet channel, and a cross over tunnel at the portion of the module adjacent the press housing, is disposed between the feeder screw housing and the press housing. The mass outlet of the press housing including a restrictor pate and means for applying a load to said restrictor plate. The use of this system is limited to the treatment of liquid-solid material with a high viscosity and containing (fine, large) particles. Such a system is not suited for the treatment of solid-liquid material feeding with a liquid concentration that may change during processing.
The restraint system existing in equipment such as those rotative extractors described in the Canadian patent CA-A-1,193,903 and in patent U.S. Pat. No. 5,344,575 does not permit the dehydration of some sludges containing materials with a high dehydration potential and with a high friction potential of the product at the exit of the equipment (cake).
In effect, such sludges due to the materials that they contain generate a high friction (fibres, material composed by relatively coarse agglomerates, etc.) at the level of the walls of the filtrating elements.
These sludges of various origins (industrial fields, cities, paste and papers, etc.) have the particularity of generating a blockage of the exit of the dehydration equipment during the process. The high friction generated favours the formation of cakes having a relatively high siccity and a very rigid consistence and textures which is comparable to a brick instead of the rigidity of a sponge at the exit of rectangular walls of the rotative extractor. This very rigid consistence results in that the cake, which has lost its elasticity, does not transfer any effort to the device for the control of the pressure at the exit of the extractor, with the consequence of frequent blockages generating a substantial loss of yield.
There was therefore a need for a single extracting system suitable for the efficient continuous treatment of various liquid-solid materials with various liquid concentration, consistency and size of solid particles.
There was also a need for an extracting system suitable for the continuous dewatering treatment of a feeding source, i.e. of a liquid solid with a composition, consistency and nature of the solid particles present in the feeding source susceptible to vary during the processing.